1. Field of the Invention
The present invention relates to an image forming apparatus for transferring, onto a transferring material held on a transferring material holding member, a toner image obtained by developing over an image bearing member with a toner on the basis of electrophotographic system or an electrostatic recording system, or to an image forming apparatus for performing a primary transfer onto an intermediate transferring member and thereafter performing a secondary transfer onto a transferring material, and can be embodied in a copying machine, a laser beam printer, a FAX and so forth.
2. Related Background Art
The following are examples of an image forming apparatus such as a copying machine and a printer based on the electrophotographic system.
One example of the system is that a toner image formed on an image bearing member is electrostatically transferred onto an intermediate transferring member, and the toner image on the intermediate transferring member is further electrostatically transferred onto a transferring material. The transferring material onto which the toner image has been transferred is separated from a transferring material holding member, and thereafter the image is fixed. Then, after transferring the toner image onto the transferring material, the surface of the intermediate transferring member is cleaned off by a cleaning unit.
Another example of the system is that the toner image forming on the image bearing member is electrostatically transferred onto the transferring material held and carried by the transferring material holding member in such a way that the transferring material holding member electrostatically adsorbs the toner image. The transferring material onto which the toner image has been transferred is separated from the transferring material holding member, and thereafter the image is fixed. Then, the surface of the transferring material holding member after the transferring material has been separated from is cleaned off by the cleaning unit.
In the image forming apparatus adopting the intermediate transfer system described above, the toner carried on the intermediate transferring member is adhered to a belt by an electrostatic force (Coulomb attraction) and an intermolecular force (van der Waals force), etc.. The secondary transfer is a process of pulling toner particles away from the intermediate transferring member (intermediate transferring belt), wherein with a secondary transfer bias, an electric field is applied to a toner layer on the intermediate transferring member, thereby carrying the electrified toner onto the transferring material.
At this time, if an adhesion force to the intermediate transferring member locally rises for some reason, a secondary transferring property worsens at that local portion, and the image might be deteriorated.
A foreign matter adhered to the intermediate transferring member is removed normally by use of a cleaning unit for the intermediate transferring member. The cleaning unit may involve the use of the known cleaning unit such as a cleaning blade, a fur brush or bias roller cleaning and so on, wherein the foreign matter, if substantially equal to or larger than a toner size, can be removed.
In these cleaning units, if continuously used for a long period of time, a friction force with the intermediate transferring member increases. Therefore, in the case of utilizing, for instance, a blade-shaped cleaning unit, an undesirable phenomenon such as a burr, a chatter, etc. occurs.
A measure for this has hitherto been taken so as not to cause the image deterioration due to the adhesion of the foreign matter to the intermediate transferring member in a way that keeps a cleaning property for a long period of time by a method of, as known in Japanese Patent Application Laid-Open No. 11-95573, periodically forming a toner zone on the intermediate transferring member and supplying the toner zone to the cleaning unit without being secondarily transferred, thereby relieving a load on the cleaning portion and reducing a friction force.
Further, as known in Japanese Patent Application Laid-Open No. 2001-175090, there is taken a method of supplying the toner in order to avoid a damage to the cleaning blade. In a case where small-width sheets such as postcards, label sheets, etc. are continuously passed by, a large surface electric charge is selectively applied to a portion, having no small-width transferring material in a main scan direction, of the intermediate transferring belt, and therefore surface roughness of the intermediate transferring belt might increase upon receiving a discharge attack. At this time, since only roughness of a non-sheet passing portion of the intermediate transferring belt rises, an edge of the cleaning blade is burred at a portion corresponding to the non-sheet passing portion or locally damaged, resulting in a decline of the cleaning property. Hence, this method intends to relieve the load on the cleaning blade by forming the toner zone at only the non-sheet passing portion.
As explained above, there has hitherto been taken the measure for relieving the load on the cleaning unit by forming the toner zone on the intermediate transferring belt so that the cleaning unit can stably remove the foreign matter over the long period of time.
There might be, however, a case where a foreign matter that can not easily be removed by the cleaning unit described above is to be adhered, a secondary transfer property might worsen if unable to remove this type of foreign matter.
For example, if a user or a serviceman carelessly touches the intermediate transferring member, a smegma is adhered onto the intermediate transferring member. The smegma is a cortical secretion that can not be easily removed by the known cleaning method. In this case, a compatibility of the smegma with a toner base material, e.g., with a polyester resin is extremely high, and hence the adhesion force between the toner and the smegma extremely increases, with the result that the secondary transfer property locally worsens. Accordingly, if carelessly touching the intermediate transferring member, a local decrease in density assuming a fingerprinted shape occurs in the image.
In this respect, there is a scheme for avoiding the careless finger touch on the intermediate transferring member when the user or the serviceman replace the intermediate transferring member or an intermediate transferring member stretching unit by providing an intermediate transferring member protect member as known in, e.g., Japanese Patent Application Laid-Open No. 11-84985.
On the other hand, other than the case where the adhesive matter is artificially stuck to the belt as described above, there is a case in which the foreign matter is transferred from the transferring material when a specified type of transferring material is passed by.
According to the intermediate transfer system, even a full-color image formation involves only one transfer as a secondary transfer for transferring the toner image onto the sheet, and therefore a configuration of a transferring material conveying portion is less complicated than in other systems. Hence, it is possible to correspond to a wide range of transferring materials such as a label sheet, a postcard, an envelop, a tab sheet, an OHP sheet, etc. in addition to, of course, ordinary sheets ranging from a thin sheet (50 g/m2) up to an extra-thick sheet (260 g/m2).
When the thus-diversified transferring materials are passed by, these transferring materials undergo the transfer in a pressurized-state at a secondary transfer portion, so that a filler of the transferring material and a sheet powder are transferred onto the intermediate transferring member.
A large foreign matter such as the sheet powder that is on the order of several tens of microns or larger, can be collected by an intermediate transferring belt cleaner.
While on the other hand, as in the case of the filler in the transferring material, particles smaller than the toner particles might not be collectable by the cleaner. This is exemplified by, for instance, a charge inhibition agent coated over the surface of a transparency film for OHP (Over-Head Projector) (which will hereinafter be called an OHP sheet) and a glue of the envelop.
The OHP film is configured by providing, for example, a resin layer containing the charge inhibition agent on a resin base layer composed of a high-transparency PET resin, PC resin, etc. as a base material. The charge inhibition agent is contained as a filler for the purpose of improving a conveying property in the image forming apparatus by restraining an electrostatic adsorption between the OHP films and for the purpose of ensuring a preferable transferring property by adjusting a surface resistivity of the OHP film.
A measure (Japanese Patent Application Laid-Open No. 1-315768, etc.) against the OHP conveying deterioration is an adjustment of friction by making a coating layer on the surface contain a mat agent, and so on. Further, a polyethylene terephthalate film has a high surface intrinsic resistance, and hence, when trying to form the image as it is, it is required that a bias transfer potential applied to the contact transferring material be high on the occasion of transferring onto the film a toner image on the image bearing member such as a photosensitive member. Consequently, there is a case where the image deterioration occurs due to an abnormal discharge. A scheme for coping with this problem is a restraint of the surface intrinsic resistance down to a certain normal value by coating the charge inhibition agent over the film surface. This enables a charge-stuck conveying deterioration to be retrained. A multiplicity of means for adjusting the surface resistance have been proposed. A general means among those is a method of coating the charge inhibition agent over the surface of a support member. Agents exemplified as the charge inhibition agent are ion conductive agents (anionic charge inhibition agent, cationic charge inhibition agent, amphoteric charge inhibition agent, etc.), and electron conductive agents (zinc oxide, tin oxide, titanium oxide, etc.) (Japanese Patent Application Laid-Open Nos. 62-94332 and 6-75419, etc.).
Some of the charge inhibition agents given above are contained in the OHP film in the form of filling the resin layer and are coated over directly the resin film after being solved in a volatile solvent such as methyl ethyl ketone and so on.
If a surface-active agent defined as an OHP filler described above is transferred onto the intermediate transferring member, a surface energy of the intermediate transferring member decreases in a high-humidity environment. The intermediate transferring member with its surface energy decreased comes to have a decrease in toner releasing property, i.e., the toner adhesion force might increase.
On the other hand, in the case of a sheet-glued transferring material such as the envelop, the glue is eluted in the high-humidity environment and might be transferred onto the intermediate transferring member. The glue used for the envelop is an easy-to-dissolve starch glue, etc. that is frequently utilized in terms of its adaptability to the environment, and this is an easy-to-be-wettable material in the high-humidity environment.
Then, if the glue of the envelop is transferred onto the intermediate transferring member at the secondary transfer portion where the envelop is brought into contact with the intermediate transferring member, the toner adhesion force of the intermediate transferring member rises, and hence the secondary transfer property might worsen.
As explained above, when a specified transferring material is passed by, a substance causing a decrease in surface energy is transferred onto the intermediate transferring member from the transferring material, the toner adhesion force of the intermediate transferring member increases, and the secondary transfer property worsens. As a result, it might appear as the image deterioration.
On the other hand, the image forming apparatus using the transferring material holding member has, though capable of preventing a problem derived from a rise in transfer potential due to a charge-up by the charge inhibition agent used on the transparent film (OHP), a possibility of causing such a fresh problem that the charge inhibition agent is transferred onto the conveying belt from the OHP, and the image deterioration is caused by a remarkable decline of an adsorption force of the transferring material (recording material) to the conveying belt defined as a transferring material holding member. This phenomenon will hereinafter be described in detail.
FIG. 11 shows a transition of the adsorption force between the transparent film and the conveying belt when the transparent films are consecutively passed by in the image forming apparatus shown in FIG. 10. The adsorption force is, as shown in FIG. 12, obtained by pulling an A4-sized recording material P adsorbed to the conveying belt in an arrowhead direction by a spring scale and measuring a critical tensile force F when the recording material P starts sliding. It is understood from examinations by the present inventors that if the adsorption force is smaller than approximately 1 kgf, the recording material deviates from the conveying belt while the recording material is carried with the result that the image deterioration such as a color deviation, etc. occurs. It is also, however, understood that the adsorption force decreases as the transparent film is passed by and eventually becomes smaller than 1 kgf as seen in FIG. 11. The reason for this is considered such that the charge inhibition agent on the transparent film is transferred onto the conveying belt with the result that the surface resistivity of the conveying belt surface is reduced, and hence there are decreased a charge retainability on the conveying belt surface and also an electrostatic adsorption force.
Further, there is considered a case in which the charge inhibition agent is coated over only the surface or only the undersurface or both of the surfaces depending on the type of the transparent film. It is conceived that the transparent film, of which both surfaces or only the undersurface is coated with the charge inhibition agent, is easy to transfer the same agent onto the conveying belt. The transparent film, of which only the surface is coated with the charge inhibition agent, likewise brings about the same image deterioration as the above-mentioned because of the charge inhibition agent being transferred once onto, e.g., the photosensitive drum, etc. and further transferred again onto the conveying belt.
For preventing such a problem from arising, it may suffice that the charge inhibition agent adhered onto the conveying belt surface can be cleaned by the cleaning unit such as, e.g., the fur brush, etc. The cleaning unit such as the fur brush has, however, no effectiveness in the charge inhibition agent and could remove almost no charge inhibition agent.
Japanese Patent Application Laid-Open No.9-212008 discloses a configuration of changing a cleaning capability of the cleaning unit and utilizing an auxiliary cleaning unit in order to effectively remove the toner, the releasing type oil and the foreign matter such as dust, etc.. This configuration has, however, a problem in which the construction of the apparatus and the control become complicated.
As explained above, in the image forming apparatus utilizing the intermediate transferring member and the transferring material holding member, there arises such a problem that the image deterioration occurs by an influence of the transfer the charge inhibition agent and the glue from the transferring material in the case of forming the image on a specified transferring material containing the charge inhibition agent and the glue.